1. University of Science and Technology MISiS 1 Department of Semiconductor Electronics and Semiconductor Physics, , 4 Leninsky Avenue, Moscow 119049, Russia
2. Department of Physics/Centre for Materials Science and Nanotechnology, University of Oslo 2 , P.O. Box 1048, Blindern, Oslo N-0316, Norway
3. Department of Materials Science and Engineering, University of Florida 3 , Gainesville, Florida 32611